(1) Field of the Invention
The present invention relates to a vessel lid constructed of a laminated material. More particularly, the present invention relates to a vessel lid which has excellent corrosion resistance, openability and feathering resistance, and drop resistance, particularly an easily openable vessel lid.
(2) Description of the Related Art
There are offered to general use cans prepared with an easily openable top end as package-use cans which can be easily opened without employing a special gadget. For the aforementioned vessel lid metallic materials constructed of aluminum, tin or tin-free steel are used for workability so that the opening section in the vessel lid constructed of a metal sheet is outlined by a score cut to a midway depth into the metal sheet, a rivet is formed in said opening section from the vessel lid sheet per se and a pull tab is fixed with said rivet. The aforementioned vessel lid is lapped lock seamed with the flange on the can body by double crimping (the two edges are first hooked and then bumped or flattened together to form five folds of metal).
The aforementioned easily openable vessel lid requires touch-up coating on its inside surface, since the coating on the internal surface is susceptible to injuries at the scoring step or at the riveting step.
There is disclosed in Japanese Patent Application Sho-62-52045, as a vessel lid which does not require touch-up coating and yet imparts excellent corrosion resistance, an easily openable vessel lid constructed of a composite material consisting of an aluminum substrate, a biaxially drawn polyethylene terephthalate film layer having a thickness of 10 to 40 microns placed on the inside of said substrate and an epoxy-phenolic resin adhesive primer layer having a thickness of 0.3 to 3 microns which is interposed between the aluminum substrate and the film layer, which is characterized in that a score is formed in said aluminum substrate to a midway depth in the vertical direction to its surface.
There is proposed in Japanese Patent Application Sho-63-12445 the provision of an epoxy thermosetting plastic coating containing a lubricant onto the film surface for preventing formation of pinholes or cracks in the film when said composite material contacts a tool or a conveying device.
The coating consisting of a polyester film is excellent in the continuity compared with the coating formed with so-called liquid paint, is excellent in the workability which is an inherent property of any thermoplastic resin, particularly biaxially oriented one has a high mechanical strength and is excellent in the dimensional stability and the heat stability. Although the polyester film has most excellent properties among all thermoplastic films as described above, there still remained various unresolved problems for the polyester film in its application to the easily openable vessel lid.
That is to say, in cases where the metallic material for the vessel lid is a steel sheet, since steel has a higher strength than a aluminum it requires a strong power to break the score, keeping the consumer from opening the easily openable vessel lid constructed of a steel sheet, or making the customer feel it difficult to open it or even causing the tab to be broken, thus impairing the openability.
As a means to improve the openability of the easily openable vessel lid constructed of a steel sheet, it has been attempted to increase the depth of the score. It was, however, the drawback of a deep score that cracks or pinholes are formed in the polyester coating provided onto the metal surface in advance, hence the corrosion resistance is deteriorated.
Moreover, the polyester coating often suffered injuries during riveting or bonding the pull tab to the vessel lid, and re-coating of an organic coating material after the tab fixing step and the scoring step was required to a disadvantage from the productivity point of view.
Therefore, it has remained impossible to obtain an easily openable vessel lid constructed of a steel sheet which is satisfactory with respect to the productivity, corrosion resistance and openability in combination.
Moreover, even in cases where the metallic material for vessel lid has a low strength as is the case with an aluminum alloy, it requires a stronger power to open the vessel lid provided with a polyester coating than for those provided with a liquid coating, and sometimes causes feathering when the vessel lid is opened.
Although the openability can be improved by increasing the depth of the score, an attempt to deepen the score would lead to formation of cracks or pinholes in the polyester coating provided onto the metal surface in advance as is the case with the vessel lid constructed of a steel sheet, thus deteriorating the corrosion resistance. Consequently, it was requisite to go through a hardly productive step of once again providing the surface with an organic coating one by one after the scoring step, depending on the condition of the vessel lid.
The present inventors have discovered through comprehensive research on polyester films used for lamination that the formation of cracks and pinholes in fabrication of the easily openable vessel lid and occurrence of feathering in opening the vessel lid or the openability are closely related with the crystalline orientation in the film in a specific direction and the in-plane anisotropy of crystal orientation and, furthermore, discovered that the aforementioned drawbacks can be effectively eliminated by limiting the crystal orientation and in-plane anisotropy of crystal orientation within specific ranges.